1. Field of the Invention
The present invention relates to a semiconductor laser and, more particularly, to a visible-ray semiconductor laser composed of AlGaInP.
2. Description of the Prior Art
The visible-ray semiconductor laser composed of AlGaInP has been attracting particular attention of late as a light source for optical information processing, and development of various adequate structures is currently in progress to realize a lower threshold value and lateral mode control for practical use.
Relative to such a semiconductor laser of AlGaInP, there is known an improved example as disclosed in Japanese Patent Laid-open No. She 62 (1987)-26885 filed by the present applicant, wherein a p-type cladding layer on one side to be attached to a header or a heat sink is formed into a dual layer structure (hereinafter referred to as DSC structure) which consists of an AlGaInP layer and an AlGaAs layer of high thermal conductivity so as to enhance the heat radiation effect.
For the purpose of lowering the threshold value in the semiconductor laser, there is provided a ridge pattern wherein both sides of the semiconductor layer adjacent to the p-type cladding layer on the active layer are removed by etching, while a central portion thereof is left unremoved in the shape of a stripe, and an n-type current stricture layer is formed in the removed portion by epitaxial growth.
If the above-described DSC structure is applied to a semiconductor laser of such a ridge pattern, there is achievable an advantage in manufacture of a ridge-shaped region.
More specifically, in a semiconductor laser having the above DSC structure as shown in an exemplary sectional view of FIG. 4, there are sequentially formed an n-type buffer layer 2 of GaAs, an n-type cladding layer 3 of AlGaInP, a non-doped active layer 4, a p-type cladding layer 5 and a cap layer 6 on a semiconductor substrate 1 which is composed of n-type GaAs or the like having a high impurity concentration with its [100] crystal plane used as a main surface. Furthermore, the p-type cladding layer 5 has a laminated structure consisting of a first cladding layer 51 of (Al.sub.x Ga.sub.1-x)InP disposed on one side adjacent to the active layer 4, and a second cladding layer 52 of Al.sub.y Ga.sub.1-y As disposed on the reverse side.
When the ridge pattern is employed in the semiconductor laser of such a DSC structure, as shown in a schematic sectional view of FIG. 5 representing a process for manufacture of the semiconductor laser, first a striped mask 7 of SiN.sub.x or the like is deposited on a central portion of the cap layer 6, and an etching step is executed from the side of the cap layer 6 by the use of such a mask 7 and an etching liquid of sulfuric acid. As a result of such a step, the etching in progress is apparently brought to a halt at the time when the first cladding layer 51 of AlGaInP, where the corrosive action speed by the etching liquid of sulfuric acid is extremely low, has been exposed to the outside. Therefore, if the etching step is interrupted at such time, merely the cap layer 6 and the second cladding layer 52 of the p-type cladding layer 5 are selectively etched at both sides thereof which are not covered with the mask 7. In this stage, the etching is effected with the liquid entering under the two side edges of the mask 7, and the lateral surface 8 formed by such etching becomes a [111]A crystal surface.
Thereafter, as shown in a schematic sectional view of FIG. 6 (representing a state after completion of a hereafter mentioned current stricture layer 10 and subsequent withdrawal of the mask 7), an n-type current stricture layer 10 of GaAs is selectively formed, while being covered with the mask 7, in the etch-removed portion 9 by epitaxial growth based on a process of MOCVD (Metalorganic Chemical Vapor Deposition). However, due to the existence of As on the lateral surface 8 and the existence of P on the [100] crystal surface of the first cladding layer 51 formed by the etching, it is difficult to attain selective epitaxial growth completely with high reproducibility on both of such surfaces. Furthermore, since heat is applied up to 720.degree. C. or so during such epitaxial growth, P contained in the first cladding layer 51 is evaporated to consequently deteriorate the characteristics thereof, hence raising another problem in the manufacture of a high-reliability element.